In an aircraft gas turbine engine, a starter motor applies torque to the engine's shaft in order to start the engine. As the shaft starts to rotate, air is inducted into the compressor, compressed and then discharged into the combustor. Concurrently, the engine's fuel control system feeds fuel into the combustor in accordance with a pre-programmed fuel schedule in order to precisely maintain the proper fuel/air ratio in the combustor, thereby achieving a light-off condition. After light-off, the starter motor torque is augmented by torque from the engine's turbine. Before reaching idling speed of the engine, the starter motor is shut off. This operation constitutes a starting cycle of a gas turbine engine.
DC (Direct Current) motors are commonly used as starters. The torque vs. speed characteristic of DC motors is fixed when the electric power supplied to the DC motors is predetermined and unchangeable, which is typically the case when batteries are used as the electric power source. However, the resistance to rotation of the gas turbine engine during the starting cycle is variable under different conditions. The resistance elements include the inertia of the engine rotor with all rotating components of the engine, oil drag, and aerodynamic components which include the load applied to the rotor of engine by air flow inducted into the engine when the engine is rotated by the starter. Therefore, the resistance varies when engine starting conditions change. For example, temperature change will vary the oil viscosity and thereby the oil drag. Temperature and altitude changes will also vary the density of the air, resulting in changes of aerodynamic components. Thus, the starter is not always enabled to efficiently start the engine under all conditions.
This problem is more severe when an auxiliary power unit (APU) engine is concerned. APU engines are usually operated on the ground to provide pneumatic power, AC (Alternating Current) current and cooling air to the aircraft, especially before the main engine of the aircraft is started and operated at a self-sustaining level. During flight the APU engines are usually not in operation. However, in some emergency situations, APU engines are used as an emergency power unit (EPU) and are required to start at a high altitude and under cold conditions during flight, which is significantly different from the starting conditions on the ground. The conventional gas turbine engine starting system is not adapted for starting engines under such conditions.
Therefore, there is a need for a gas turbine engine starting system adapted for effectively starting a gas turbine engine under various conditions.